Microfluidic systems are flow systems miniaturized to dimensions typically as small as a few micrometers (μm). Such systems present challenges in both their design and manufacture. For example, at the level of miniaturization of typical microfluidic systems, fluid flow is predominantly laminar. Laminar flow may also be found in other fluidic systems, such as those including viscous fluids or having low flow rates. Accordingly, the challenges presented by working in laminar flow regimes are not limited to microfluidic systems.
Fluid flow conditions in microfluidic systems and otherwise are typically represented by a dimensionless number known as the Reynolds Number (Re). The Reynolds number is defined as the ratio of inertial forces to viscous forces within a fluid. A low Re (e.g., <100) is associated with laminar flow, while a high Re (e.g., >2000) is associated with turbulent flow. For example, turbulent flow typically develops at Re>2000 in a circular pipe. Fluid flow in microfluidic systems is typically characterized by very low Re (e.g., <10), representing stable, laminar flow. Flow lines of laminar flow—as opposed to turbulent flow—do not diverge in simple geometries, and several streams in a capillary may flow in parallel, mixing only by diffusion.
Recent developments in microfluidic systems have been motivated in large part by the possibility of fabricating compact, integrated devices for analytical functions such as genomic analysis, diagnosis and sensing.